1. Field of the Invention
The present invention relates to silicon controller rectifiers (SCRs) and, more particularly, to an SCR with a fuse that prevents latchup.
2. Description of the Related Art
A silicon controlled rectifier (SCR) is a device that provides an open circuit between a first node and a second node when the voltage across the first and second nodes is positive and less than a trigger voltage. However, when the voltage across the first and second nodes rises to be equal to or greater than the trigger voltage, the SCR snaps back.
When the SCR snaps back, the SCR allows a large current to flow between the first and second nodes at a much lower voltage as long as a minimum current or a minimum voltage, known as a holding current or a holding voltage, is maintained. If the current flowing between the first and second nodes falls below the holding current, or the voltage across the first and second nodes falls below the holding voltage, the SCR again provides an open circuit between the first and second nodes.
As a result of these characteristics, SCRs are used with electronic circuits to protect the electronic circuits from an electro-static discharge (ESD) pulse when an ESD pulse is unintentionally applied to the pins of a chip that houses the electronic circuits. An ESD pulse can be unintentionally generated when a chip is handled prior to being attached to a printed circuit board.
When an ESD pulse is generated, a very high potential is momentarily placed on a pin while the chip is otherwise powered off. If another pin is grounded, a very large current can flow from the high potential pin through circuitry in the chip to the grounded pin. If the pins are not ESD protected, the current can destroy the circuitry in the chip.
FIG. 1 shows a schematic diagram that illustrates a portion of a conventional chip 100. As shown in FIG. 1, chip 100 includes a first pin 110 and a second pin 112. In addition, chip 100 includes an electronic circuit 114 and an SCR 116 that are both connected to first pin 110 and second pin 112.
Thus, during normal operation, SCR 116 provides an open circuit between first pin 110 and second pin 112. However, when first pin 110 receives a voltage spike that equals or exceeds the trigger voltage of SCR 116, such as when an ungrounded human-body contact occurs, SCR 116 provides a low-resistance current path from first pin 110 to second pin 112, thereby protecting electric circuit 114 from damage.
An SCR ideally operates within an ESD protection window that has a maximum voltage that is defined by the destructive breakdown level of the devices that are electrically connected to a pin, and a minimum voltage that is defined by any DC bias voltage that is present on the pin during normal operation. The trigger voltage of the SCR is then set to a value that is less than the maximum voltage of the window, while the holding voltage is set to a value that is greater than the minimum voltage of the window.
It is often difficult to fabricate an SCR that has a holding voltage which is greater than the DC bias voltage that is placed on the pin during normal operation. As a result, many SCRs are fabricated with a holding voltage that is less than the DC bias voltage. However, when the holding voltage is less than the DC bias voltage, the chip is subject to a condition known as latchup.
Latchup occurs when the SCR misfires and turns on during normal operation. When the holding voltage is less than the DC bias voltage, and the SCR turns on and remains turned on during normal operation, the SCR pulls the voltage on the pin down to the holding voltage which, in turn, effectively disables the entire operation of the circuitry with the chip.
For example, if an SCR has a holding voltage of 1.0V and the chip places a DC bias voltage of 1.8V on a pin during normal operation, then the SCR pulls the voltage on the pin down to 1.0V when the SCR misfires and turns on during normal operation. In addition, since the DC bias voltage of 1.8V is greater than the holding voltage of 1.0V, the DC bias voltage ensures that once the SCR misfires and turns on, the SCR remains turned on until power is removed from the chip. Thus, unless the circuitry on the chip can operate with 1.0V, the circuitry is disabled.
In addition, if the SCR sinks a large current while latched up, the large current can lead to excessive heating that can burn out the circuitry on the chip. As a result, there is a need for an approach that prevents an SCR from latching up when the SCR, which has a holding voltage less than the DC bias voltage, misfires and turns on during normal operation.